Respect declareExternallyReferenced for non-interface types

[alexmojaki]

Fixes #525

[bcherny]

This won't quite work as expected, I think -- what happens in this case?:

export const input = {
  type: 'object',
  definitions: {
    a: {
      title: 'a',
      type: 'string'
    }
  },
  properties: {
    a: {
      $ref: '#/definitions/a',
    }
  },
}

Can you spec out how this should behave a bit better? As designed, I'd expect declareExternallyReferenced: false to still emit a declaration for A in this case, since A was declared locally.

We might want a way to pipe through whether a dereference was external or not. See resolver.ts.